1. Field of the Invention
The present invention relates to a hard coating. It relates further to a workpiece coated by such hard coating and to a method of coating a workpiece by such a hard coating.
2. Description of the Prior Art
A predominant feature of aluminum oxide is an extremely high thermodynamic stability and a comparatively high hardness. As thermodynamic stability thereof is understood its resistance in an aggressive environment also at high temperatures. Apart from its use on parts subject to wear in aggressive mediums aluminum oxide is used as coating material, especially for tools used for machining or cutting operations. Coatings of aluminum oxide are further used in optical and electronic applications. Generally known is the deposition of aluminum oxide coatings by means of chemical or physical gas phase deposition, CVD or PVD. In case of a chemical deposition aluminum chloride is brought to react with water vapor. At reaction temperatures of 1000.degree. C. coatings grow at a rate of maximal 0.3 .mu.m/h such as disclosed in the paper "Aluminium-oxid-CVD mit einem AlCl.sub.3 /O.sub.2 Reaktionsgemisch" (Aluminum oxide CVD by a AlCl.sub.3 /O.sub.2 reaction mixture) of H. Altena et al., R & HM, March 1987. The method described in this paper is used for a coating of hard metals. The coatings grow with crystalline structure whereby generally the .alpha.- or .kappa.-modification of aluminum oxide is produced, see also S. Vuorinen et al., "Characterization of .alpha.-Al.sub.2 O.sub.3, .kappa.-Al.sub.2 O.sub.3 and .alpha.-.kappa. multi-oxide coatings on cemented carbides", Thin Solid Films, 193/194 (1990) 536-546.
Because the .alpha.-modification has the higher thermal stability it also has a better wear resistance. Further, because the forming of nuclei on the substrate proceeds quite inhomogeneously it is necessary, in order to achieve a coherent coating, to deposit initially an intermediate layer such as is taught in M. Kornmann et al., "Nucleation of Alumina Layers on TiC and Cemented Carbides by Chemical Vapor Deposition", J. Cryst. Growth, 28 (1975), 259-262. In technical processes generally a TiC intermediate layer is used.
If a metal organic composition is used as a Al-donor instead of an aluminum chloride, such as aluminum-triisopropylate or aluminum-trimethyl, the depositing temperature can be lowered to about 420.degree. C. as disclosed in J. E. Carnes et al., "Self-Healing Breakdown Measurements of Pyrolytic Aluminum Oxide Films on Silicon", Journal of Applied Physics, 42 (11), 4350-4356 (1971). This method finds application e.g. in the production of electronic circuits. An amorphous, i.e. not crystalline aluminum oxide layer is produced.
For the physical deposition of aluminum oxide, vapor deposition and sputtering methods can be used. Coatings which are produced by these methods find optical and electronic applications.
In case of sputtered coatings an amorphous structure is produced at a temperature of the workpiece below 500.degree. C. Only above a workpiece temperature of 1100.degree. C. the .alpha.-modification segregates from aluminum oxide such as disclosed in J. A. Thornton, "Structure and Heat Treatment Characteristics of Sputter-Deposited Alumina", Am. Ceram. Soc. Bull., 56 (5), 504-508, 512 (1977).
The reactive plasma enhanced vapor deposition of aluminum oxide is described in R. F. Bunshah et al., "Alumina Deposition by Activated Reactive Evaporation", Thin Solid Films, 40, 211-216(1977). Also here crystalline coatings in a .alpha.-modification are found only at a workpiece temperature&gt;1100.degree. C. The mechanical properties of these coatings are, however, poor. A coating hardness of 500 to 800 HV is disclosed. Also, these coatings are described as being porous and as adhering poorly.